The invention generally relates to systems and methods for controlling the infusion of medical fluids and, more particularly, to a fluid flow restrictor placed in an infusion line to achieve a more uniform rate of flow through that line while reducing the possibility of contaminants being conducted.
Infusion of fluids is one of the most widespread procedures in medicine. Infusion systems deliver liquid therapeutic substances (e.g. drugs in solution, saline, nutrients) to patients typically through veins and arteries, but also into interstitial spaces as well. Every infusion is driven by some source of pressure. Two common sources of fluid pressure for causing the infusion of medical fluids into a patient are gravity and positive pressure infusion pumps. The medical fluids are typically delivered through sterile, single-use, disposable fluid administration sets that comprise tubing and a cannula or catheter and perhaps an administration port or ports along the tubing for the infusion of additional medical fluids.
Infusion systems operating by gravity typically use a container of the medical infusion fluid suspended above the patient. In gravity infusion the pressure for infusing the medical fluid is produced by the very weight of the medical fluid itself. In practice, this is effected by suspending the container higher than the patient. Then the pressure of the fluid produced by gravity is high enough to overcome the counter pressure of the patient's circulatory system and thus allows for infusion of the medical fluid into the patient. In such a system, the magnitude of the pressure depends on the height of the container. However, as the fluid level in the container decreases, the pressure decreases and the rate of flow decreases. A varying rate of flow is undesirable for some medications as it has been found that a uniform flow rate of medical fluid has a more predictable treatment effect on the patient.
Furthermore, the requirement that the container of medical fluid must be suspended above the patient in a gravity system has made such systems impractical for use with ambulatory patients. Many surgical procedures today may be completed on an “out patient” basis. In many cases, the surgical procedure can be completed in less than a few hours and the patient may leave the health care facility in an “ambulatory” state. Yet the infusion of medical fluid after completion of the procedure is necessary for the patient's well being. The same is true for patients who have been released after extended stays at health care facilities. The continued infusion of medical fluids may be necessary for them. Whether that medical fluid is pain medication, nerve block (anesthetic), chemotherapy, or other, the continued infusion of that fluid into the patient may be necessary.
Because of this more and more common situation of an ambulatory patient with a continuing need for infusion, ambulatory infusion pumps have been developed. Such pumps can be carried by the patient at a position lower than the patient's heart, such as on the patient's belt because they use a positive pressure source. The pressure source of these pumps is strong enough to force the infusion fluid into the patient regardless of the location of the pump in relation to the patient's heart. However, such ambulatory infusion pumps must be relatively inexpensive since they are personal in nature and it is desirable that they be disposable by the patients after the treatment has been completed. Because of this need to create a lower cost and disposable ambulatory pump, many do not produce flow rates that have the desired level of uniformity.
In most cases, the mechanism used to apply pressure to the medical fluid to achieve infusion is not linear across its entire range of operation. In many cases, the amount of pressure exerted on the medical fluid when the pump is full differs from the amount of pressure when the pump is almost empty. Without further intervention, the flow rate will be correspondingly variable and non-uniform. Thus some manufacturers include fluid flow restrictors in the fluid line leading to the patient. Such flow restrictors “restrict” the flow rate of medical fluid to the patient to a level that will achieve the desired therapeutic effect. Cost considerations require that such flow restrictors be produced at low cost, yet be as accurate as possible.
To achieve the advantages of a portable ambulatory pump, several types of mechanisms have been suggested. Because of the needs to control cost and limit complexity, and to function for an ambulatory patent, a mechanical power source has proven to be more desirable.
Of the numerous mechanical structures that have been used as a pumping chamber in portable infusion pumps, one structure is of particular interest. This structure comprises an elastomeric membrane that is stretched beyond its at-rest configuration by the loading of medical fluid into it. The membrane's tendency to return to the contracted configuration provides the necessary mechanical power to move fluid through a tube to a patient. As the membrane tends to return to its at-rest configuration, the medical fluid within it is expelled out of the membrane, through the administration set, and into the patient. An elastomeric pumping mechanism has several features which make it attractive for such an application. Firstly, an elastomeric structure is relatively inexpensive to manufacture. Secondly, it has an operational simplicity that enhances its appeal for use in devices which are to be operated by lay persons.
However, an elastomeric membrane does not address the problem encountered at the end of a pumping cycle that is caused by the inability of an elastomeric membrane to maintain a constant pressure within the fluid chamber as the membrane approaches its unstretched state. With some, the membrane snaps back to its at-rest configuration as it nears that state. This results in a period of non-uniformity in the flow rate. As is well known, constant pressure within the pumping chamber during a pumping operation from beginning to end is very much desired to obtain a uniform dispensing rate.
One portable pump that takes these features into account is the ReadyMED pump made and distributed by the ALARIS Products division of Cardinal Health, San Diego, Calif. It includes a housing that stretches an elastomeric membrane into its region of nonlinear elasticity. To do this, the housing is formed with a surface that has a predetermined contour that is circumscribed by a periphery. The elastomeric membrane is then attached to this periphery to position the membrane over and across the contoured surface of the housing. This stretches the membrane into its region of nonlinear elasticity, and creates a fluid chamber between the surface of the housing and the elastomeric membrane. When the fluid chamber is filled with medical fluid, the stretched membrane generates a substantially uniform pressure on the fluid within the chamber for a uniform discharge of the fluid from the chamber.
There are several types of flow restrictors available. One example is a tube clamp that may be set by the nurse to pinch the infusion tubing partially closed to achieve a desired flow rate. Although these are simple devices and relatively inexpensive, most tubing will react to the continued application of pressure from a pinch clamp and the flow rate will vary. A more precise means of restricting the flow is desired.
Another device used as a flow restrictor is a capillary tube. With the principal flow restrictor comprising a capillary element, the extent to which the restrictor limits the fluid flow rate is determined by the length and cross-sectional area of the capillary element itself. These dimensions are selected based on the input pressure from the pressure source to deliver the liquid medicament at a predetermined flow rate. If the capillary element is to be maintained within the housing, it becomes difficult to substantially lengthen the capillary element without requiring a re-design of the housing. To avoid such a re-design, the internal diameter of the capillary element may be varied; however, with the small internal diameters utilized in the flow restrictor, such as a capillary of about 0.041 mm (0.0016 inch) in diameter, it becomes difficult to consistently manufacture capillary elements having the required precise internal diameter. It should be noted that variances in diameter can average out over the length of the capillary element but as noted above, the length of the element is limited when it is disposed within the housing, thereby limiting the extent to which the effect of variances along the length of the capillary element can be minimized.
Flow rates such as about 48 milliliters (“ml”) in twenty-four hours or even slower flow rates are desirable in certain circumstances. As an example only, it is sometimes desirable to deliver 36 ml, 48 ml, or 60 ml of a medical liquid in three, four, or five days.
A further limitation with present infusion capillary tube systems is that because of the diameter involved, capillary tube fluid flow restrictors can become easily clogged. If an infusion line becomes clogged, fluid flow will slow or stop. Such a stoppage in treatment is undesirable for a patient who may depend on the medication for pain relief or for other reasons.
Currently, some pumps having elastomeric drive devices use glass capillary tubes with a very small inside diameter and a macroscopic outside diameter. This type of tube has several drawbacks associated with its use. It is difficult to obtain an adequate seal with the outside diameter of the glass because of the smooth nature of glass. Due to the cleaving operation by which the glass is cut, the outside edges of the outside diameter are sharp and can create particulate matter that may enter the fluid flow stream. This particulate can occlude the inside of the capillary, especially when the glass capillary is inserted into a fitting or a flexible sleeve. This problem has been resolved in the past by roughing or flame polishing the edges of the glass but which could again result in particulate that can occlude the capillary or which can create additional labor steps. Finally, since the cut edge of the glass capillary and the exit hole are on the same plane, these restrictors are also more likely to become clogged by particulate which comes to rest on this cut edge in close proximity to the exit hole. Thus, it is advantageous to have a fluid flow restrictor that is capable of reducing the flow rate while being less prone to clogging.
Hence those skilled in the art have recognized a need for a fluid flow restrictor that is less prone to clogging. Yet a further identified need is for a flow restrictor that can be manufactured more cost effectively and can use interchangeable parts with restrictors of differing sizes. The invention fulfills these needs and others.